1. Field of the invention
The invention in general relates to antenna systems and more particularly to a wideband antenna system which generates one or more electronically steerable beams.
2. Description of related art
Various electronic systems exist which utilize an antenna, having a certain beam pattern, for the transmission and/or reception of microwave energy. For many of these systems it is a requirement that they function at relatively wideband operation. For example, in the communications art, the amount of information which can be transmitted depends upon the bandwidth of the system, with the greater the bandwidth, the greater the data rate. In the radar field, for example, improved range resolution may be achieved with wideband operation.
Many such electronic systems utilize an electronically steerable array (ESA) antenna system to augment, or eliminate the physical mechanical steering of the antenna structure. Basically, in an ESA arrangement, the antenna is comprised of a plurality of individual antenna elements and means are provided for each antenna element to alter the phase of the microwave signal going to (or coming from) the antenna element in order to steer the antenna beam in a certain off-axis direction, depending upon the phase alterations.
Typical phase altering devices include phase shifters and time delay circuits. In general, due to the requirement for constant time delay for electronic steering of the antenna beam, phase shifters do not perform well for wideband operation.
Time delay circuits can provide the necessary phase alteration across the entire face of the antenna structure to achieve desired off-axis directionality. A typical electronic time delay circuit includes a plurality of delay line stages each with a different delay time insertable into the microwave signal path. The insertion or removal of a delay segment is accomplished by miniature switches the states of which are governed by a control circuit.
For example, miniature electronic microwave switches in common use include the gallium arsenide (GaAs) field effect transistor (FET) and the GaAs pin diode. Both of these devices operate at extremely high speeds and can achieve switching rates measurable in nanoseconds.
For some applications however, the GaAs FET has objectionably high resistance when closed and high capacitance when open, resulting in a relatively low cut-off frequency, for example, 600 GHz. The pin diode exhibits a higher cut-off frequency of around 2 THz, however it, along with the GaAs FET, exhibits an objectionably high capacitance in the off state. For this reason these switches are often operated with a separate shunt inductor resonant with the capacitance, at the operating frequency.
This added inductor advantageously increases the impedance of the switch in the off condition, however this arrangement objectionably lowers the operating bandwidth of the overall switch device. In addition, the pin diode switch requires an expenditure of current to hold it in the on condition, as well as a means to inject and remove this current. This bias coupling current lowers the effective cut off frequency of the pin diode switch. For example this current may be in the order of around 1 to 10 milliamps per diode switch. There may be, however, thousands of such switches in an entire ESA system resulting in an objectionably high power consumption, measurable in kilowatts.
The antenna system of the present invention utilizing delay lines with small switches having significantly lower capacitance in the off state and lower resistance in the on state results in a system with a high cut-off frequency and true time delay across a wide band of frequencies.